The present invention relates to a solid ultrasonic delay line for providing a delayed signal by propagating an ultrasonic signal through a solid delay medium such as of glass, and more particularly to such a solid ultrasonic delay line of a shear mode in which a dispersive mode signal is suppressed to a practically sufficient degree.
Glass delay lines are widely used in color television receivers, video disk reproducing units, video tape recorders and the like. One such glass delay line is illustrated in FIG. 1 of the accompanying drawings. As shown in FIG. 1, a delay medium D has two major faces 1, 2 and five end faces 3, 4, 5, 6, 7 intersecting the major faces 1, 2. An input transducer 8 is bonded to the end face 3 and an output transducer 9 is bonded to the end face 4. An ultrasonic energy which is produced by the input transducer 8 is propagated in the delay medium along reflected paths shown by the arrows toward the output transducer 9. The input transducer 8 serves to convert an electrical signal into an ultrasonic signal which is polarized parallel to the major faces 1, 2 of the delay medium D so that an ultrasonic signal of a shear mode will be generated. The shear mode ultrasonic signal is then fed to the output transducer 9 and reconverted thereby into an electrical signal.
The delay line of the above construction is characterized in that it can well be mass-produced and is capable of easily processing spurious signals. For the propagation of only nondispersive mode (zero mode) ultrasonic signals, it is necessary to employ transducers having a thickness equal to that of the delay medium. Stated otherwise, the input and output transducers 8, 9 have boundary surfaces 10, 11, 12, 13 lying flush with the two major faces 1, 2 of the delay medium D. For propagating of only zero mode ultrasonic signals, it is also necessary that the thickness of the delay medium be less than five times or preferably half the wavelength of the ultrasonic wave propagating in the delay medium.
The foregoing theory is described in IRE TRANSACTIONS, July 1960, pages 35 through 43. The mass production of delay lines is disclosed in U.S. Pat. No. 3,581,247, and removal of spurious signals is described in Japanese Patent Publication No. 47-27574 which claims Convention priority of Dutch patent application No. 6816005 filed Nov. 9, 1968. Particularly, the technology as described in U.S. Pat. No. 3,581,247 that delay lines as shown in FIG. 1 are mass-produced by slicing a block of glass to which transducers have been bonded has been employed by major manufacturers in the world because of its low cost production process. The thickness of the glass delay medium is generally selected to be about twice the wavelength of propagating signals for providing necessary strength.
As described above, the IRE TRANSACTIONS referred to above describes that the thickness of the delay medium should be less than half of the propagating wavelength for the propagation of only zero mode ultrasonic signals. However, no literature is available which would give a technical explanation as to why the delay line as illustrated in FIG. 1 can propagate zero mode ultrasonic signals even if the thickness of the delay line is several times greater than the wavelength of the propagating signal.
The present inventors have analyzed the above phenomenon and reached the following conclusion: The delay line as shown in FIG. 1 has plural mode paths according to prescribed rules in conformity with the conventional theory as long as the delay medium has a thickness of .lambda./2 or more, but the input and output transducers are incapable of transmitting and receiving signals other than zero mode signals. This arises out of the fact that the boundary surfaces of the transducers lie exactly flush with the major faces of the delay medium. The reason why the transducers cannot transmit and receive signal other than zero mode signals if the boundary surfaces of the transducers are flush exactly with the major faces of the delay medium will be described below.
FIG. 3 shows by way of example the manner in which a mode wave with n=3 propagates through a medium D. Waves W.sub.I, W.sub.II are expressed by: EQU W.sub.I =ce.sup.j(ky+.gamma.x) e.sup.jwt ( 1) EQU W.sub.II =ce.sup.-j(ky-.gamma.x) e.sup.jwt ( 2)
where k is a propagation constant in the direction of y and .gamma. is a propagation constant in the direction of x.
A composite wave W.sub.III is then given as follows: EQU W.sub.III =c{e.sup.j(ky+.gamma.x) +e.sup.-j(ky-.gamma.x) }e.sup.jwt ( 3)
The above equation is indicative of the mode wave with n=3.
In FIG. 3, the direction of Y is vertical, the direction of X is horizontal, and the direction of Z is normal to the sheet of the drawing. The waveforms are all indicative of displacements in the direction of Z. The major faces of the delay medium are denoted by 1, 2 and the boundary surfaces of the transducer by 12, 13. The equation (3) can be modified as follows: EQU W.sub.III =C' cos kye.sup.-j(.gamma.x-wt) ( 4)
When y=.+-.b, a boundary condition is ##EQU1## Therefore ##EQU2## and hence k=n.sup..pi. /b' (n=0, 1, 2 . . . ). Accordingly, the wave III is found to be a mode wave advancing in the direction of X and vibrating according to the expression e.sup.-j(.gamma.x-wt) while forming a standing wave as defined by C' cos (n.sup..pi. /b)y in the direction of Y.
The formation of a standing wave in the direction of Y in a zone is illustrated in FIG. 4. It is assumed now that a transducer as defined by A-B-C-C is bonded to the delay medium at a point X=L thereon. The wave W.sub.III as expressed by the equation (4) goes through the transducer until it is reflected at the point X=L as the boundary surfaces of the transducer lie flush with the major faces of the delay medium. A wave W.sub.IV through the transducer can be defined as follows: ##EQU3## For the sake of brevity, k, .gamma. is presumed to be equal to values in the delay medium.
From a boundary condition ##EQU4## and hence L=0, .pi., 2.pi., . . . n.pi. If L=0 at the point x=L, .gamma.(L-T)=.pi. at a point x=L-T because of the relationship T=.lambda./2 between the resonant frequency of the transducer and the thickness T. From the equation (5), standing waves are generated in the transducer along the direction of Y as shown in FIG. 6 and along the direction of X as shown in FIG. 5. The standing waves have waveforms such that they vibrate exteriorly of the transducer, but no output is produced by the output transducer since electromotive forces are cancelled out as a whole. Stated otherwise, the delay line with the boundary surfaces (A-B, C-D) of the transducer lying flush with the major faces of the delay medium will not operate in a dispersive mode. If the delay medium is thicker, that is, if the standing wave as shown in FIG. 4 has a wavelength of a few .lambda. or more (experimentally, 5 or more), it will be difficult to obtain effective standing waves, and the delay line will operate in a dispersive mode.
It is now assumed that a transducer as defined by A'-B'-C'-D' is bonded. The dispersive mode wave progressing through the medium, as expressed by the equation (4), does not satisfy the boundary condition in the direction of Y ##EQU5## and no standing wave is generated in the direction of Y, and only a standing wave in the direction of X is produced as shown in FIG. 5. Accordingly, this delay line operates in a dispersive mode. The same holds true with a transducer as defined by A-A"'-C"'-D.
It is apparent that with a transducer A"-B"-C"-D, the delay line operates well in a dispersive mode. A transducer as defined by A"-B"'-C-D is practically effective as a shear mode transducer, as disclosed in Japanese Patent Laid-Open Publication No. 50-134350, and the fact that a delay line with such a transducer will not operate in a dispersive mode can clearly be explained only by the foregoing theory.
According to the prior art as described in the above various publications, the thickness of the delay medium should be five times or more greater than the wavelength of an ultrasonic signal propagating therethrough, and preferably be twice larger than the wavelength for obtaining a practical solid delay line, and this requirement is experimentally established. In fact, those glass delay mediums which find widest use in color television receivers have a thickness ranging from 1 mm to 1.2 mm with a central frequency of about 3.6 MHz. Since the wavelength of the ultrasonic signal propagating in the delay medium is about 0.6 mm, the thickness of the medium is selected to be about 1.5 to 2 times the wavelength.
The glass delay lines of such a central frequency can have a transmission band of about 2 MHz. However, glass delay lines used in video cameras and other broadcasing equipment are required to have as wide a transmission band as possible, ranging from 5 MHz to 10 MHz. Therefore, the central frequency is also selected to be in the range from 10 MHz to 30 MHz.
With the central frequency being higher, the wavelength of the signal propagating in the medium becomes shorter; for example, the maximum thickness of the medium should be selected to be about 0.3 mm for the central frequency of 30 MHz. Such a glass delay medium is practically infeasible as it can easily be broken. Those delay mediums having a thickness five times or more greater than the wavelength and fabricated according to the conventional process suffer from severe spurious signals and hence poor characteristics.
Therefore, prior delay lines having high central frequencies are limited to a construction having a glass delay medium about 5 mm thick with square or circular transducers attached and employing a bulk wave. Such a delay line is quite costly as it cannot be mass-produced by way of a slicing process as described above, and cannot be reduced in weight and size.